The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Reverse osmosis systems typically use one or more membrane housings that have one or more membranes therein that are used to extract an essentially pure fluid from a solution. The desalination reverse osmosis membranes receive feed fluid from brackish or sea water and extract fresh water therefrom. Fresh water is extracted or separated when the pressure of the feed fluid exceeds the osmotic pressure of the fluid which allows permeate or product fluid to cross the semi-permeable reverse osmosis membrane. The fluid that is left on the input side to the membrane becomes higher in salt concentration because fresh water that travels through the membrane does not include the salt. The water that passes through the membrane is referred to as a permeate. The pressure required to produce fresh water is proportional to the concentration of the total dissolved solids (TDS) in this feed solution within the reverse osmosis housing. For typical ocean water, the concentration is about 35,000 parts per million (ppm) and the corresponding osmotic pressure is about 450 pounds per square inch (psi) (3,102 kPa). For 70,000 ppm feed fluid, the osmotic pressure approximately doubles to 900 psi (about 6,205 kPa). A typical seawater reverse osmosis system uses a series of membranes that recover up to about 45% of the fresh water and generate about 55% concentrate brine from the original volume of seawater. The net driving pressure (NDP) equals the feed pressure minus the osmotic pressure. The net driving pressure is the pressure energy available to drive pure fluid across the membrane.
Referring now to FIG. 1, a membrane channel 10 is illustrated between two membrane sheets 12. The channel 10 includes an inlet 14 and an outlet 16. An amount of permeate 18 represented by the droplets has permeated from the channel 10 through the membrane sheets 12. As the feed fluid that enters the inlet 14 and progresses through the membrane channel 10, the concentration of dissolved solids increases as the permeate 18 is extracted. The higher number of droplets of permeate 18 toward the inlet 14 indicate that permeate production is higher toward the inlet 14 and decreases toward the outlet 16. Because of the increasing totaled dissolved solids and the corresponding reduction in the net driving pressure, less permeate is extracted from the channel 10.
Referring now to FIG. 2, the relationship of the feed pressure, osmotic pressure, the feed total dissolved solids, the permeate rate and the net driving pressure is illustrated for a membrane channel of a reverse osmosis system with about 45% recovery handling of the seawater. As is illustrated, the feed pressure is about 860 psi (5929 kPa) and loses about 10 psi (69 kPa) over the life of the channel. The osmotic pressure at the start of the channel is about 450 psi (3,100 kPa) and rises to about 820 psi (5653 kPa) due to the increase in total dissolved solid of the feed. The feed total dissolved solid begins at about 35,000 ppm and rises to about 63,000 ppm at the end of the channel. The net driving pressure begins at about 500 psi (3450 kPa) and decreases to about 50 psi (350 kPa). The permeate flow rate decreases to negligible at the end of the channel 10.
Referring now to FIG. 3, a batch reverse osmosis system 30 is illustrated. The batch reverse osmosis system 30 is used to treat a volume of feed fluid. The process repeatedly passes an initial volume of feed fluid through the reverse osmosis membranes and removes permeate until a desired level of concentration of total dissolved solids or a specific amount of permeate has been produced. The batch reverse osmosis system 30 has a feed reservoir or source 32 that communicates fluid to a charge pump 34. The charge pump 34 communicates fluid through a valve 36 and into an inlet 38 of a tank 40. The valve 36 is open during filling of a batch tank 40 and is closed after the tank 40 is filled with feed fluid. The tank 40 may include an air vent 41 for releasing displaced air as the tank 40 is filled and drawing in air as the tank 40 is emptied. A drain valve 42 is coupled to a port 44 for draining the processed fluid into a brine tank 46 as will be described in more detail below.
An outlet port 50 communicates fluid from the tank 40 to a high pressure pump 52 through pipes 51 and 53. The high pressure pump 52 increases the pressure of the fluid from the tank 40 and communicates the fluid to the membrane housing 54 that has a membrane 56 therein. A permeate pipe 58 drains permeate that passes through the membrane 56. The permeate pipe 58 is in communication with a permeate tank 60 that collects the permeate that passes therethrough. A brine pipe 62 communicates brine concentrated fluid through a valve 64 to a port 66 in the tank 40.
A controller 70 coupled to a concentration sensor 72 monitors the process and the concentration of the fluid within the tank 40 to end the process when the fluid within the tank 40 reaches a predetermined concentration. The controller 70 may also be used to control the various valves including valve 36, the valve 64 and the pumps including the high pressure pump 52 and the charge pump 34. In the process, feed fluid is provided to the tank 40 through the charge pump 34 and open valve 36. When the tank 40 is filled, the charge pump 34 is powered off and the valve 36 is closed. As the tank 40 is filling, air is vented from the tank through the air vent 41. Drain valve 42 is also closed during the filling of the tank 40 through pipe 38. During batch processing, the high pressure pump 52 is controlled to provide pressure. The valve 64 is also opened during batch processing to circulate the concentrated brine back to the tank 40. During batch processing, fluid from the tank 40 leaves the port 50 and enters the pipe 51 whereby the high pressure pump 52 increases the pressure and provides the desired pressure to the membrane housing 54 through pipe 53. Permeate exits the membrane housing through the pipe 58. The control valve 64 is adjusted to achieve a desired flow rate and depressurized brine fluid returns to the tank 40 through port 66.
As the batch of fluid within the tank is processed, concentrated brine is recirculated back to the tank 40 which increases the concentration of the fluid within the tank 40. As the fluid becomes increasingly concentrated, the pressure output by the high pressure pump 52 is increased. The recirculation of the fluid from the tank 40 to the high pressure pump 52 through the membrane housing 54, brine pipe 62 and the valve 64 continues until the sensor 72 measures the ending concentration.
Once the desired concentration has been achieved, the high pressure pump 52 is stopped and the concentrate within the tank 40 is drained through the drain valve 42 which is opened to drain the fluid into the brine tank 46. Thereafter, the drain valve 42 is closed and the charge pump 34 is activated and the valve 36 is open to provide a fresh batch of feed fluid to the tank 40.
Referring now to FIG. 4, another prior reverse osmosis system 30′ is illustrated in further detail. The same components illustrated in FIG. 3 are provided with the same reference numerals. In this example, an energy recovery device such as a turbocharger 74 having a turbine portion 74T and a pump portion 74P is used to recover at least a portion of the energy of the pressurized brine concentrate stream in the brine outlet pipe 62. That is, the brine from the membrane housing 54 is communicated to the turbine portion 74T of the turbocharger 74. The turbine 74T rotates the pump portion 74P to increase the pressure within the input line 78 to the membrane housing 54. The depressurized brine fluid returns to the tank 40 through the inlet port 66 and the valve 64. The high pressure pump 52 can operate at a lower pressure because of the boost provided by the turbocharger 74. By reducing the pump power, the added heat into the fluid is reduced. This also eliminates cooling equipment to maintain the fluid temperature to a desired batch temperature.
Referring now to FIG. 5, a similar example to that illustrated in FIG. 3 is set forth. In this example, another high pressure pump 80 is coupled between the fill reservoir 32 and the input port 38 to the tank 40. This configuration may be referred to as a “semi-batch” reverse osmosis system 30″. In this example, the charge pump 34 transfers feed fluid from the reservoir 32 to entirely fill the tank 40. The valve 36 is closed. The high pressure pump 80 is used to pump feed into the fluid tank 40 from the reservoir 32. The pump 52 is engaged so that as feed is injected into the tank 40 through the feed pipe 51 and the feed fluid input pipe 53, the pressure quickly rises due to the incompressibility of the fluid. The pressure reaches the point where the permeate exits the membrane housing 54 through the permeate outlet pipe 58 into the tank 60. The permeate flow equals the rate of flow from the high pressure pump 80. As permeate is extracted, the pressure in tank 40 increase to overcome the increasing osmotic pressure in order to maintain the permeate flow into the permeate tank 60. The pump 52 circulates fluid from the tank 40 through the membrane housing 54 and back to the tank 40. Concentration sensor 72 senses the concentration of the fluid within the tank 40 and when a concentration has reached a concentration limit, the high pressure pump 80 is shut down and valve 42 is opened to allow the fluid within the tank 40 to drain into the brine tank 46. The next batch begins by opening the valve 36 and increasing the amount of fluid within the tank 40 until it is full wherein the valve 36 is closed and the high pressure pump 80 is activated to feed fluid into the tank 40 during the permeate production as described above.